Transmitter and a method for transmitting data

ABSTRACT

An excitation reference source (F C ) is split through a 90 degree splitter. One output from the splitter is fed to the LO port of a mixer. Data is fed to the mixer&#39;s IF port and causes PRK modulation of the LO port&#39;s signal. The output of the mixer at the RF port is a PRK modulated quadrature signal. This is attenuated and added back onto the reference by a zero degree combiner ready for transmission to the transponder.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 09/582,341, filed on Aug. 22, 2000. U.S. patent applicationSer. No. 09/582,341 is a National Stage Application of InternationalPatent Application PCT/AU98/01077, filed on Dec. 24, 1998. InternationalPatent Application PCT/AU98/01077 is incorporated herein by reference.

FIELD

The invention relates to a transmitter and a method for transmittingdata.

The invention has been developed primarily for the field of radiofrequency identification (RFID), and more particularly to a method fortransmitting data to a transponder with a single antenna, and will bedescribed hereinafter with reference to that application. This inventionhas particular merit when applied to passive transponders where highspeed data transmission is desirable.

BACKGROUND

Hitherto, high speed data has been transmitted to RFID transponders bymodulation of the excitation field. Generally pulse position modulationwith 100% depth amplitude modulation of the excitation field is used.The excitation field is turned off for short intervals which aredetected by the transponder's processing circuitry. To achieve high datarates while maintaining the transmission of power the intervals must beshort and the duty cycle low. Typically a duty cycle of 10% is used andthe intervals are 1 μs long and the average time between intervals is 10μs. Short intervals such as these have a wide bandwidth. Accordingly,both the interrogator and the transponder require low Q factor, widebandwidth antennae to transmit and receive the data. Low Q factorantennae are not energy efficient and, as such, the interrogator antennawill consume more power than a high Q factor antenna. Moreover, forpassive transponders a stronger excitation field is required tocompensate for the less efficient antenna.

Additionally, regulations governing the magnitude of electromagneticemissions place upper limits on the strength of excitation fields thatcan be used and the allowable bandwidth of an excitation field. The widebandwidth of the prior art pulse, modulation data results in limitationsbeing placed on the maximum excitation field strength.

DISCLOSURE

It is an object of the invention, at least in the preferred embodiment,to overcome or substantially ameliorate at least one of thedisadvantages of the prior art.

According to one aspect of the invention there is provided a method fortransmitting data from a first antenna, said method including the stepsof:

providing a carrier signal;

imposing a low level phase modulation on the carrier signal inaccordance with a data signal to create a modulated signal

providing the modulated signal to said first antenna for transmission.

According to a second aspect of the invention there is provided atransmitter including:

a first antenna;

oscillator means for providing a carrier signal; and

mixing means for imposing a low level phase modulation on the carriersignal in accordance with a data signal to create a modulated signal,the mixing means also providing the modulated signal to the firstantenna for transmission.

Preferably, the modulated signal is received by a second antenna whichin response thereto, produces a first signal which is provided toreceiver means, the receiver means deriving a second signal indicativeof the data signal. Even more preferably, the first signal is used topower the receiver means.

In a preferred form, the modulated signal includes the sum of thecarrier signal and an attenuated quadrature carrier signal which ismodulated with the data signal. This form of modulation is describedherein as phase jitter modulation (PJM).

In a preferred form the antenna is a tunable coil. Preferably also, boththe first and second antennas have a high Q factor.

According to another aspect of the invention there is provided anidentification system including a transmitter as described above.

Preferably, the system is for identifying luggage.

BRIEF DESCRIPTION OF THE DRAWINGS

The prior art and a preferred embodiment of the invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic illustration of a prior art transponder circuit;

FIG. 2 illustrates representative waveforms associated with the priorart circuit of FIG. 1;

FIGS. 3( a) to 3(c) are frequency spectra associated with the waveformsof the prior art circuit of FIG. 1;

FIGS. 4( a) and 4(b) are phasor diagrams for waveforms produced inaccordance with the invention;

FIG. 5( a) to 5(c) are frequency spectra associated with the invention;

FIGS. 6( a) and 6(b) respectively illustrate methods of encoding anddecoding data in accordance with the invention;

FIG. 7 is a schematic illustration of a preferred circuit for encodingthe data signal for transmission; and

FIG. 8 is a schematic illustration of a preferred circuit for decodingthe data signal in the transponder.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Passive RFID transponders that incorporate a single antenna areinterrogated by an interrogator using an excitation field. This field isreceived by the transponder's antenna and the voltage induced on theantenna is rectified and used to power the transponder. Often it isnecessary for the transponder to receive data transmitted from itsinterrogator. For single antenna transponders the received messages mustbe received by the same antenna that is used to receive the excitationsignal used to power the transponder. In prior art systems theexcitation signal is amplitude modulated to convey messages from theinterrogator to the transponder.

FIG. 1 shows a prior art transponder where the antenna L is tuned by acapacitor C and data is transmitted to the transponder by amplitudemodulation. The voltage V1 induced in the transponder's antenna coil ismagnified by the antenna's tuning, rectified by the rectifiers andstored on the DC storage capacitor Cdc for use by the transponder'selectronic circuits. The antenna voltage is peak level detected by thediode envelope detector D1, C1 and R1 to give the envelope voltage V2.

FIGS. 2( a) and 2(b) illustrate waveforms associated with the prior artcircuit of FIG. 1. More particularly, FIG. 2( a) shows the excitationvoltage V1 with amplitude intervals to giving pulse position modulation.To deliver the maximum power to the transponder, a low duty cycle isused, typically 10:1. FIG. 2( b) shows the envelope of the voltage V2induced in the antenna. The antenna's transient response results in afinite rise and fall time for V2. The transient time of the antenna mustbe sufficiently short to allow narrow pulses to pass without significantdistortion. The antenna's transient response time constant Ts andbandwidth BW are related by Ts=1/(BW·π). Accordingly, to pass shortpulses the bandwidth of the antenna must be broad. For example, to pass1 μs pulses a bandwidth of at least 1 MHz is required.

FIGS. 3( a) to 3(c) are frequency spectra associated with the prior artcircuit of FIG. 1. FIG. 3( a) shows a typical data spectrum. For data at100 kbps the first zero of the frequency spectrum occurs at 100 kHz.FIG. 3( b) shows the data spectrum when encoded as pulse positionmodulation PPM where narrow low duty cycle pulses are used. The spectrumfor this type of encoding is much broader than the original dataspectrum. For 1 μs pulses with a 10:1 duty cycle the first amplitudezero of the frequency spectrum occurs at 1 MHz. FIG. 3( c) shows thespectrum of the excitation signal when modulated with the PPM signalwhose spectrum is shown at FIG. 3( b). The modulated spectrum is doublesided and accordingly, for 1 μs pulses with a 10:1 duty cycle the widthof the main spectral lobe is 2 MHz. Clearly the bandwidth of the PPMmodulated excitation signal is much broader than the original dataspectrum.

To pass the inherently broad band PPM excitation signal both theinterrogator and transponder antenna must have a wide bandwidth.Consequently, the interrogator and transponder antennae must have a lowQ and will operate with a low efficiency. In the interrogator thegeneration of 100% amplitude modulated PPM requires that excitationsignal be completely quenched for each pulse. This requires a wide bandlow efficiency antenna. Narrow band antennae would operate with highefficiency but are unable to respond to the narrow amplitude pulses ofPPM. Similarly, the transponder antenna bandwidth must be broad bandenough to pass the modulated excitation signal. Broad band antennae areinherently low Q and are poor collectors of energy from an excitationfield.

In this preferred embodiment of the invention data is imposed as a lowlevel signal having a modulated quadrature component. Most preferablythis modulation is phase modulation although in other embodiments use ismade of amplitude modulation. In the present embodiment the low levelsignal appears as a tiny phase jitter in the excitation field. There isno change in the amplitude of the excitation field and hence thetransmission of power to the transponder is unaffected. This form ofmodulation will be termed phase jitter modulation or, for convenience,PJM.

There are many methods of producing small modulated phase shifts. Forexample, by passing the signal through a phase shifter such as an RC ortuned circuit, or through a variable length delay line.

In this embodiment, to produce the signal at the interrogator, a smallportion of the excitation signal is phase shifted 90 degrees to give aquadrature signal. This is then PRK modulated with the data signal andadded back onto the original excitation signal before being transmittedto the transponder. The resultant signal can be amplitude limited toremove any residual amplitude component. At the transponder these tinyphase shifts in the excitation induce corresponding antenna voltagephase shifts that are unaltered by any circuit impedances or powerregulation circuitry connected to the transponder's antenna.

FIG. 4( a) is a phasor diagram of the excitation signal Fc and themodulated quadrature signal PRK. The amplitude of the respective signalsare given by their phasor lengths. The phase deviation THETA caused bythe modulated quadrature signal is, for low level signals, extremelysmall and is given by:THETA=arctan(2×Mag(PRK)/Mag(Fc))

For a 40 dB attenuated PRK signal THETA=1.2 degrees and for a 60 dBattenuated PRK signal THETA=0.12 degrees. Both of these are extremelysmall phase deviations of the excitation signal.

Phase quadrature modulation is recovered using a local oscillator (LO)signal, with a fixed phase with respect to the excitation signal, todown convert the modulated data to baseband in a mixer or multiplier. Inthe transponder the LO signal must be derived from the modulatedexcitation signal. The preferred method of extracting a LO signal fromthe modulated excitation signal uses a Phase Locked Loop PLL in thetransponder to generate the LO signal. The LO signal is generated by alow loop bandwidth PLL which locks to the original excitation signal'sphase but is unable to track the high speed modulated phase shifts. Thequadrature data signal is down converted and detected in a mixer ormultiplier driven with the LO signal. Depending upon the type of phasedetector used in the PLL, and the propagation delays through thecircuit, the phase of the LO with respect to the excitation signal canbe anywhere between 0° and 360°. If a conventional XOR phase detector isused in the PLL then the output of the PLL oscillator will be atnominally 90 degrees to the excitation signal and will be in phase withthe data modulated phase quadrature signal. A 90° phase between the LOand the excitation signal is not necessary for the effective detectionof quadrature phase modulation. An XOR mixer has a linear phase tovoltage conversion characteristic from 0° to 180° and 180° to 360°.Hence it gives the same output amplitude irrespective of the phase angleexcept around 0° and 180° where there is a gain sign change.

The average output voltage DC level from a mixer is a function of theaverage phase difference between its inputs. It is more convenient forcircuit operation for the average output to be around midspan and hencean LO with a phase angle of around 90° is more convenient. The phase ofthe LO signal can be simply adjusted using fixed phase delay elements.Hence a 0° or 180° phase detector can be used and a further 90°(roughly) of phase shift can be achieved with a fixed delay element.

FIG. 4( b) is a phasor diagram of the modulated excitation signal and aquadrature local oscillator signal in the transponder used to demodulatethe data signal. The local oscillator signals phase is at 90 degreeswith respect to the excitation signal's phase.

For phase modulation the data bandwidth is no broader than the originaldouble sided data bandwidth. When attenuated the level of the modulateddata spectrum is extremely low with respect to the excitation signalamplitude making conformance to regulatory emission limits significantlyeasier than with the prior art.

FIGS. 5( a) to 5(c) are representative frequency spectra that explainthe operation of the invention. More particularly, FIG. 5( a) is atypical data spectrum. For data at 100 kbps the first zero of thefrequency spectrum occurs at 100 kHz. FIG. 5( b) is a representativefrequency spectrum of the data when modulated onto a quadrature versionof the excitation signal. The spectrum for this type of modulation isthe same as the double sided spectrum of the original data spectrum. Inthe invention the modulated quadrature signal is attenuated and added tothe original excitation signal. FIG. 5( c) shows the spectrum of theexcitation signal Fc plus the attenuated modulated quadrature signalwhose spectrum is shown in FIG. 5( b). The attenuation level is given bythe difference between the amplitude of the excitation signal and theamplitude of the data sidebands.

Since the spectrum of the transmitted excitation signal is equal to theoriginal double sided data spectrum, narrow band high Q interrogator andtransponder antennae are used to respectively transmit and receive themodulated excitation signal. Consequently, the interrogator's excitationantenna operates with high efficiency and the transponder's antennalikewise receives energy with high efficiency. In other embodiments useis made of low Q antennae.

FIGS. 6( a) and 6 (b) show methods of modulating and demodulatingaccording to this invention. Turning first to FIG. 6( a), the portion ofthe main excitation signal is phase shifted 90 degrees to produce aquadrature signal. The quadrature signal is then modulated with data.The preferred form of modulation is phase reverse keying PRK. The PRKmodulated quadrature signal is attenuated and then added back to themain excitation signal. Although shown in a particular order thesequence phase shift, modulation and attenuation are done in otherorders in alternative embodiments. This method of modulation produceslow level data side bands on the excitation signal where the sidebandsare in phase quadrature to the excitation signal. The data signalappears as a low amplitude phase jitter on the excitation signal. Insome embodiment the signal is further amplitude limited to remove anyresidual amplitude component.

FIG. 6( b) illustrates a method for demodulating the data modulated onto the excitation signal. A LO signal is generated by a low loopbandwidth phase lock loop PLL. The PLL locks on to the excitationsignals phase and is unable to follow the high speed phase jitter causedby the data modulation. For the standard PLL phase detector the PLLoscillator will lock at a fixed phase with respect to the excitationsignal's phase. This oscillator signal is then used as a LO todemodulate the quadrature sideband data signal in the multiplier. A lowpass filter LPF filters out high frequency mixer products and passes thedemodulated data signal.

FIG. 7 shows an example circuit for encoding the data signal fortransmission. An excitation reference source Fc is split through a 90degree splitter. One output from the splitter is fed to the LO port of amixer. Data is fed to the mixer's IF port and causes PRK modulation ofthe LO port's signal. The output of the mixer at the RF port is a PRKmodulated quadrature signal. This is attenuated and added back onto thereference by a zero degree combiner ready for transmission to thetransponder.

FIG. 8 shows an example circuit for decoding the data signal in thetransponder. The transponder antenna voltage is squared up by a schmitttrigger, the output of which feeds a type 3 PLL. A type 3 phase detectoris a positive edge triggered sequence phase detector which will drivethe PLL oscillator to lock at 180° with respect to the input phase. Witha low loop bandwidth the PLL is able to easily filter off the sidebandson the input signal. The output of the schmitt is passed through a chainof invertors designed to add a fixed delay to the input signal. Thedelay is approximately chosen so that the phase of the output from thedelay chain is not 0° or 180° with respect to the LO. A preferred phasevalue is 90° for circuit convenience. The output of the VCO acts as theLO to demodulate the Phase Jitter Modulated data. The data isdemodulated in an exclusive OR gate, the output of which is low passfiltered and detected with a floating comparator.

Although the invention has been described with references to a specificexample it will be appreciated by those skilled in the art that it maybe embodied in many other forms.

1. A device adapted to receive a modulated signal and derive therefrom adata signal, the device comprising: an antenna adapted to receive themodulated signal and, in response thereto, produce a first signal; meansfor deriving, from the first signal, a local oscillator signal; meansfor deriving a second signal by passing a further portion of the firstsignal through a delay means; and wherein the local oscillator signaland the second signal are used to demodulate the first signal and obtainan indicative data signal.
 2. The device as claimed in claim 1, whereina phase locked loop is used to derive the local oscillator signal. 3.The device as claimed in claim 2, wherein the phase locked loop is a lowloop bandwidth phase locked loop.
 4. The device as claimed in claim 1,wherein a mixer is used in demodulating the first signal.
 5. The deviceas claimed in claim 1, wherein a multiplier is used in demodulating thefirst signal.
 6. The device as claimed in claim 1, wherein the firstsignal is used as a source of power for the device means.
 7. The deviceas claimed in claim 1, wherein the device is a Radio FrequencyIdentification Device.
 8. The device as claimed in claim 1, wherein thedevice is a passive device.
 9. The device as claimed in claim 1, whereinthe device is a transponder.
 10. A method of demodulating a modulatedsignal received by a device and deriving therefrom a data signal, themethod comprising the steps of: receiving the modulated signal andproducing a first signal; deriving, from the first signal, a localoscillator signal; deriving a second signal by passing a further portionof the first signal through a delay means; and demodulating the firstsignal using the local oscillator signal and the second signal to obtainan indicative data signal.
 11. The method as claimed in claim 10,wherein a phase locked loop is used to derive the local oscillatorsignal.
 12. The method as claimed in claim 11, wherein the phase lockedloop is a low loop bandwidth phase locked loop.
 13. The method asclaimed in claim 10, wherein a mixer is used in demodulating the firstsignal.
 14. The method as claimed in claim 10, wherein a multiplier isused in demodulating the first signal.
 15. The method as claimed inclaim 10, wherein a low pass filter is used to filter out high frequencysignal components and pass the demodulated data signal.
 16. The methodas claimed in claim 10, wherein the first signal is used as a source ofpower for the device.
 17. A method of demodulating a modulated signalreceived by a device and deriving there from a data signal, the methodcomprising the steps of: receiving the modulated signal and inducinginto an antenna of the device, an antenna voltage signal; amplifying theantenna signal; providing a portion of the amplified signal to a phaselocked loop to filter off sidebands and creating a first signal; passinganother portion of the amplified signal through a delay means andcreating a second signal; and XORing the first and second signals toprovide indicative data.
 18. The method as claimed in claim 17, whereinthe antenna signal is amplified by squaring the signal by a Schmitttrigger device.
 19. The method as claimed in claim 17, wherein the phaselocked loop is a low bandwidth phase locked loop device.
 20. The methodas claimed in claim 17, wherein the data is detected using a floatingpoint comparator device.